1. Field of the Invention
This invention relates to a binder suitable for obtaining ceramic molded green body. More specifically, high strength, high density and high uniformity binders, suitable for obtaining ceramic green tape which is soft, has high strength and excellent surface smoothness, are disclosed.
2. Description of the Prior Art
Ceramics have recently been extensively employed in various applications such as electronic materials, magnetic materials, optical materials, high temperature materials and the like. From these various applications, improvements in the properties and characteristics of the ceramics are necessary and have been eargerly sought. For example, with respect to such properties as thermal, electrical, and mechanical, products having higher density and higher uniformity are greatly desired. In the fields of mechanical parts, electrical parts and the like, products of greater size and of more complicated shapes are favored. In order to realize these requirements, a variety of studies have been conducted both with respect to the starting ceramic powder material and the method for molding.
In the past, as to the method for improving the thermal, electric, mechanical, optical and like properties of ceramic products, there has been proposed a method for upgrading the purity of the starting material powder as well as a method for employing a powder having a small particle diameter. However, difficulties are encountered when employing these methods. For example, such methods reduce the plasticizing of substances derived from impurities in the starting material or an increase in the particle surface area which, in turn, requires a greater amount of the organic binder.
On the other hand, when making a larger sized product or one of a more complicated design, the a molded green body from which the product is produced, requires higher strength when the molded green body (non-sintered product) is handled or treated. According to conventional techniques, an increased amount of an organic binder has been employed.
However, to increase the amount of the organic binder relative to the starting material powder has problems in the following areas:
1. Problems in the Debindering Operation
The molded green body is subjected to a debindering operation prior to sintering, but if the amount of the organic binder is too great at that time, large amounts of heat and gas are generated, and thus there is a risk of cracking due to explosion etc. and, in addition, a longer time for debindering is required.
2. Problems of Impurities
The addition of a large amount of an organic binder can give rise to the incorporation of impurities or an increase of the carbonized residue after sintering of the molded green body, and thereby greatly reducing the purity of the final product.
3. Problems Relating to Dimensional Accuracy
With an increase in the amount of the organic binder, the degree of shrinkage is much greater when the molded green body is sintered and thus the dimensional accuracy of the final product is reduced.
In order to solve these problems, binders such as polyvinyl alcohol (hereinafter referred to as PVA), methyl cellulose, vinyl acetate polymer emulsions illustrated in the comparative examples described hereinbelow have been studied. However, binders which can provide a high strength, a high density and a high uniform molded green body by the addition of small amounts thereof have not as yet been found.
For producing ceramic green tape (or sheet), there has gerally been employed a process which comprises dissolving a binder such as a polyvinyl butyral resin in an organic solvent such as methyl ethyl ketone, butyl acetate, and alkyl alcohols, mixing with it a ceramic powder for many hours to process it into a slip, followed by deaerating, processing it into a sheet on a carrier film using a blade having a predetermined thickness, and drying by heating to form a green tape.
However, the above described conventional molding method employing an organic solvent has the following problems:
(1) Environmental problems and problems of exposure to humans created by the use of organic solvents. PA1 (2) Potential for explosion and fire during the handling of the organic solvents. PA1 (3) The material cost of the solvent for producing green tape is expensive. PA1 (4) There is the necessity to use equipment which is explosion-proof and at the same time provide a device for recovering the organic solvent. This results in the plant cost being expensive.
In order to solve these problems, there has been proposed a molding method of employing a water-soluble organic polymer such as PVA as a binder.
Although this method has the advantage that it can solve most of the above-described problems, it has the drawback in that prior to the stage for molding the green tape, the ceramic powder tends to form a flocculate structure in an aqueous slip and thus the viscosity of the slurry deviates greatly from Newtonian flow. Additionally, the flowability of the slip and the dispersibility of the ceramic powder are poor, and thus a uniform green tape having high surface smoothness cannot easily be obtained.
The use of the water-soluble polymers such as PVA also creates further problems. For water-soluble polymers there is no plasticizer which has high plasticizing efficiency. Also, there is the problem that when a plasticizer such as glycerin is employed in a large amount in order to impart softness to the green tape, the surface of the green tape becomes tacky due to migration of the plasticizer.